Speaking at a pro-gun conference, Mr Trump questioned how the US “has $40 billion to send to Ukraine” but cannot ensure security in schools.
The meeting of the National Rifle Association (NRA), the largest US gun group, is taking place in Houston.
The intervention comes days after 21 people died in a Texas school shooting.
“Before we nation-build the rest of the world, we should be building safe schools for our own children in our own nation,” Mr Trump said in Friday’s speech, drawing loud applause.
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